Naturally occurring extracellular matrices (ECMs) are used for tissue repair and regeneration. One such ECM is small intestine submucosa (SIS). SIS has been used to repair, support, and stabilize a wide variety of anatomical defects and traumatic injuries. Commercially-available SIS material is derived from porcine small intestinal submucosa that remodels the qualities of its host when implanted in human soft tissues. Further, it is taught that the SIS material provides a natural matrix with a three-dimensional microstructure and biochemical composition that facilitates host cell proliferation and supports tissue remodeling. SIS products, such as Oasis material and Surgisis material, are commercially available from Cook Biotech, Bloomington, Ind.
An SIS product referred to as RESTORE™ Orthobiologic Implant is available from DePuy Orthopaedics, Inc. in Warsaw, Ind. The DePuy product is described for use during rotator cuff surgery, and is provided as a resorbable framework that allows the rotator cuff tendon to regenerate itself. The RESTORE™ Implant is derived from porcine small intestine submucosa that has been cleaned, disinfected, and sterilized. Small intestine submucosa (SIS) has been described as a naturally-occurring ECM composed primarily of collagenous proteins. Other biological molecules, such as growth factors, glycosaminoglycans, etc., have also been identified in SIS. See Hodde et al., Tissue Eng. 2(3): 209-217 (1996); Voytik-Harbin et al., J. Cell Biochem., 67:478-491 (1997); McPherson and Badylak, Tissue Eng., 4(1): 75-83 (1998); Hodde et al., Endothelium, 8(1):11-24 (2001); Hodde and Hiles, Wounds, 13(5): 195-201 (2001); Hurst and Bonner, J. Biomater. Sci. Polym. Ed., 12(11) 1267-1279 (2001); Hodde et al., Biomaterial, 23(8): 1841-1848 (2002); and Hodde, Tissue Eng., 8(2): 295-308 (2002), all of which are incorporated by reference herein. During seven years of preclinical testing in animals, there were no incidences of infection transmission form the implant to the host, and the RESTORE™ Implant has not decreased the systemic activity of the immune system. See Allman et al., Transplant, 17(11): 1631-1640 (2001); Allman et al., Tissue Eng., 8(1): 53-62 (2002).
While small intestine submucosa is available, other sources of submucosa are known to be effective for tissue remodeling. These sources include, but are not limited to, stomach, bladder, alimentary, respiratory, or genital submucosa, or liver basement membrane. See, e.g., U.S. Pat. Nos. 6,379,710, 6,171,344, 6,099,567, and 5,554,389, hereby incorporated by reference. Further, while SIS is most often porcine derived, it is known that these various submucosa materials may be derived from non-porcine sources, including bovine and ovine sources. Additionally, the ECM material may also include partial layers of laminar muscularis mucosa, muscularis mucosa, lamina propria, stratum compactum and/or other tissue materials depending upon factors such as the source from which the ECM material was derived and the delamination procedure.
For the purposes of this invention, it is within the definition of a naturally occurring ECM to clean, delaminate, and/or comminute the ECM, or even to cross-link the collagen fibers within the ECM. It is also within the definition of naturally occurring ECM to fully or partially remove one or more sub-components of the naturally occurring ECM. However, it is not within the definition of a naturally occurring ECM to separate and purify the natural collagen or other components or sub-components of the ECM and reform a matrix material from the purified natural collagen or other components or sub-components of the ECM. While reference is made to SIS, it is understood that other naturally occurring ECMs (e.g., stomach, bladder, alimentary, respiratory, and genital submucosa, and liver basement membrane), whatever the source (e.g., bovine, porcine, ovine) are within the scope of this disclosure. Thus, in this application, the terms “naturally occurring extracellular matrix” or “naturally occurring ECM” are intended to refer to extracellular matrix material that has been cleaned, disinfected, sterilized, and optionally cross-linked. The terms “naturally occurring extracellular matrix” and “naturally occurring ECM” are also intended to include ECM foam material prepared as described in copending U.S. patent application Ser. No. 10/195,354 entitled “Porous Extracellular Matrix Scaffold and Method”, filed concurrently herewith.
The following patents, hereby incorporated by reference, disclose the use of ECMs for the regeneration and repair of various tissues: U.S. Pat. Nos. 6,187,039; 6,176,880; 6,126,686; 6,099,567; 6,096,347; 5,997,575; 5,968,096; 5,955,110; 5,922,028; 5,885,619; 5,788,625; 5,762,966; 5,755,791; 5,753,267; 5,711,969; 5,645,860; 5,641,518; 5,554,389; 5,516,533; 5,445,833; 5,372,821; 5,352,463; 5,281,422; and 5,275,826.
The manipulation of scaffold pore size, porosity, and interconnectivity is of emerging importance in the field of tissue engineering (Ma and Zhang, 2001, J. Biomed Mater Res. 56(4): 469-477; Ma and Choi, 2001, Tissue Eng., 7(1):23-33), because it is believed that the consideration of scaffold pore size and density/porosity influences the behavior of cells and the quality of tissue regenerated. In fact, several researchers have shown that different pore sizes influence the behavior of cells in porous three-dimensional matrices. For example, it has been demonstrated in the art that for adequate bone regeneration to occur scaffold pore size should to be at least 100 microns (Klawitter et al., 1976, J Biomed Mater Res, 10(2):311-323). For pore sizes and interconnectivity less than that, poor quality bone is regenerated, and if pore size is between 10-40 microns bone cells are able to form only soft fibro-vascular tissue (White and Shors, 1991, Dent Clin North Am, 30:49-67). The current consensus of research for bone regeneration indicates that the requisite pore size for bone regeneration is 100-600 microns (Shors, 1999, Orthop Clin North Am, 30(4):599-613; Wang, 1990, Nippon Seikeigeka Gakki Zasshi, 64(9):847-859). It is generally accepted that optimal bone regeneration occurs for pore sizes between 300-600 microns.
Similarly, for the regeneration of soft orthopaedic tissues, such as ligament, tendon, cartilage, and fibro-cartilage, scaffold pore size is believed to have a substantial effect. For example, basic research has shown that cartilage cells (chondrocytes) exhibit appropriate protein expression (type II collagen) in scaffolds with pore sizes of the order of 20 microns and tend to dedifferentiate to produce type I collagen in scaffolds with nominal porosity of about 80 microns (Nehrer et al., 1997, Biomaterials, 18(11):769-776). More recently, it has been shown that cells that form ligaments, tendons, and blood vessels (fibroblasts and endothelial cells) exhibit significantly different activity when cultured on scaffolds with differing pore sizes ranging from 5 to 90 microns (Salem et al., 2002, J Biomed Mater Res, 61(2):212-217).
Copending U.S. application Ser. No. 10/195,354 entitled “Porous Extracellular Matrix Scaffold and Method”, DEP-747), filed contemporaneously herewith and hereby incorporated by reference, describes methods for making ECM foams wherein the porosity is controlled. Using the methods so described, ECM foams are made having the desired porosity for a particular application.
In some applications, it is also desirable to control the rate of resorption of the scaffold. It is known in the art to make implantable three-dimensional synthetic scaffolds with controlled porosity and controlled resorption rates. See, e.g., U.S. Pat. Nos. 6,333,029 and 6,355,699, hereby incorporated by reference. These synthetic foams may be isotropic in form, or may be anisotropic, providing various gradient architectures.
In addition to synthetic foams, it is known that resorption rates of an implant may be controlled by providing a synthetic portion comprising a perforated or non-perforated sheet or a mat with a woven, knitted, warped knitted (i.e., lace-like), nonwoven, or braided structure. It is understood that in any of the above structures, mechanical properties of the material can be altered by changing the density or texture of the material. The fibers used to make the reinforcing component can be for example, monofilaments, yarns, threads, braids, or bundles of fibers. These fibers can be made of any biocompatible material. In an exemplary embodiment, the fibers that comprise the nonwoven or three-dimensional mesh are formed of a polylactic acid (PLA) and polyglycolic acid (PGA) copolymer at a 95:5 mole ratio. Illustrated examples of the synthetic portion also include 90/10 PGA/PLA, 95/5 PLA/PGA, and polydioxanone (PDO) nonwoven mats, and perforated thin sheets of 60/40 PLA/PCL (polycaprolactone) or 65/35 PGA/PCL.
A variety of biocompatible polymers can be used to make fibers for the synthetic portion. Examples of suitable biocompatible, bioabsorbable polymers that could be used include polymers selected from the group consisting of aliphatic polyesters, poly(amino acids), copoly(ether-esters), polyalkylenes oxalates, polyamides, poly(iminocarbonates), polyorthoesters, polyoxaesters, polyamidoesters, polyoxaesters containing amine groups, poly(anhydrides), polyphosphazenes, biomolecules and blends thereof. For the purpose of this disclosure aliphatic polyesters include but are not limited to homopolymers and copolymers of lactide (which includes lactic acid, D-,L- and meso lactide), glycolide (including glycolic acid), ε-caprolactone, p-dioxanone (1,4-dioxan-2-one), trimethylene carbonate (1,3-dioxan-2-one), alkyl derivatives of trimethylene carbonate, δ-valerolactone, β-butyrolactone, γ-butyrolactone, ε-decalactone, hydroxybutyrate (repeating units), hydroxyvalerate (repeating units), 1,4-dioxepan-2-one (including its dimer 1,5,8,12-tetraoxacyclotetradecane-7,14-dione), 1,5-dioxepan-2-one, 6,6-dimethyl-1,4-dioxan-2-one 2,5-diketomorpholine, pivalolactone, α,α-diethylpropiolactone, ethylene carbonate, ethylene oxalate, 3-methyl-1,4-dioxane-2,5-dione, 3,3-diethyl-1,4-dioxan-2,5-dione, 6,8-dioxabicycloctane-7-one, copolymers, and polymer blends thereof. Other synthetic polymers are known in the art and may be used within the scope of this disclosure.
The particular polymer may be selected depending on one or more of the following factors: (a) bio-absorption (or bio-degradation) kinetics; (b) in-vivo mechanical performance; and (c) cell response to the material in terms of cell attachment, proliferation, migration and differentiation and (d) biocompatibility. With respect to the bio-absorption kinetics, it is known to control resorption rates by selection of the polymer or copolymer. By way of example, it is known that a 35:65 blend of ε-caprolactone and glycolide is a relatively fast absorbing polymer and a 40:60 blend of ε-caprolactone and (L)lactide is a relatively slow absorbing polymer. Optionally, two or more polymers or copolymers could then be blended together to form a foam having several different physical properties.
In some orthopaedic applications, it is desirable to combine the tissue remodeling properties of ECM with the controlled resorption properties of synthetic foams, mats, or sheets. Thus, methods are provided for making porous scaffolds for the repair or regeneration of a body tissue, wherein the scaffolds comprise an ECM component and a synthetic portion. According to one illustrative embodiment, there is provided a method of making an implantable scaffold for repairing damaged or diseased tissue. The method includes the steps of suspending pieces of an ECM material in a liquid and mixing a polymer solution into the liquid. The mixture is formed into a mass and, subsequently, the liquid is driven off so as to form interstices in the mass. In another embodiment, the method includes suspending pieces of an ECM material in a liquid and forming a mass. A polymer mat, for example, a mesh or nonwoven, is coated with the ECM material, and, subsequently, the liquid is driven off, forming a foam having a combination of mechanical and biological features.
In one specific implementation of an exemplary embodiment, the liquid is driven off by lyophilizing the ECM and synthetic material and the liquid in which they are suspended. In such a manner, the liquid is sublimed thereby forming the interstices in the mass.
The material density and pore size of the scaffold may be varied by controlling the rate of freezing of the suspension. The amount of water into which the pieces of extracellular matrix material are suspended may also be varied to control the material density and pore size of the resultant scaffold. Furthermore, as discussed above, the resorption rate may be controlled by varying the synthetic polymer structure or composition.
In accordance with another exemplary embodiment, there is provided an implantable scaffold for repairing or regenerating tissue prepared by the process described above.
Thus, one aspect of this disclosure is directed to a method of making an implantable scaffold for repairing or regenerating body tissue, the method comprising the steps of suspending ECM material in a liquid to form a slurry, adding a synthetic portion to the slurry to make an ECM/synthetic composition, freezing the composition to form crystals therein, and driving off the crystals to form a foam. In one illustrated embodiment, the ECM is comminuted. In another illustrated embodiment, the liquid is water, the crystals are ice, and the crystals are driven off by lyophilization.
In another aspect of this disclosure an implantable scaffold for repairing or regenerating body tissue is provided, the scaffold comprising a porous ECM foam and a synthetic mat imbedded therein.
Yet another aspect is an implantable scaffold comprising a mass of ECM intermixed with a fibrous synthetic portion in a composition dried to have a desired porosity.
Still, another aspect of this invention is an implantable scaffold comprising a porous foam comprising ECM and a synthetic portion distributed within the foam.
The above and other features of the present disclosure will become apparent from the following description and the attached drawings.